Conventional leather processing involves four important operations, viz., pre-tanning, tanning, post tanning and finishing. It includes a combination of single and multi-step processes that employs as well as expels various biological, organic and inorganic materials as described by Germann (Science and Technology for Leather into the Next Millennium, Tata McGraw-Hill Publishing Company Ltd., New Delhi, p. 283, 1999). Beam house processes (liming and reliming) employ lime and sodium sulfide and purifies the skin matrix by the removal of hair, flesh and other unwanted materials. Various application methods include pit, paddle, drum and painting on flesh side. After this stage, the hide/skin is termed as pelt. Deliming, bating and pickling processes prepare the skin for subsequent tanning. Tanned skin matrix further retanned to gain substance, fatliquored to attain required softness and dyed to preferred shades.
Generally, liming-reliming process liquors contribute to 50-70% of the total biochemical oxygen demand (BOD) and chemical oxygen demand (COD) load from a tannery wastewater and 15-20% in the case of total solids (TS) load as reported by Aloy et al (Tannery and Pollution, Centre Technique Du Cuir: Lyon, France, 1976). Apart from this, a great deal of solid wastes containing lime sludge, fleshings, and hair are generated. The extensive use of sulfide bears unfavorable consequences on environment and the efficacy of effluent treatment plants as reported by Colleran et al (Antonie van Leeuwenhoek, 67, 29, 1995).
Several lime and sulfide free liming methods have evolved during the past century. Bose and Dhar (Leather Science, 2, 140, 1955; 21, 39, 1974) have reviewed the use of enzymes such as proteolytic, amylolytic, etc from various sources namely animal, mold, bacterial and plant for dehairing hides and skins. However, these methods include the use of lime. Rosenbusch (Das Leder, 16, 237, 1965) has reported the use of chlorine dioxide for dehairing. Morera et al (Journal of the Society of Leather Technologists and Chemists, 81, 70, 1997) have studied the use of hydrogen peroxide in alkaline medium for dehairing by oxidation mechanism. However, the reduction in pollution load especially COD is not significant. Sehgal et al (Journal of the Society of Leather Technologists and Chemists, 80, 91, 1996) have developed a non-enzymatic sulfide free dehairing process using 1% nickel carbonate, 1% sodium hydroxide, 5% lime and kaolin along with water by painting. However, disposal or recovery of nickel compounds poses serious health problems. Schlosser et al (Journal of the Society of Leather Technologists and Chemists, 70, 163, 1986) have reported the use of lacto-bacillus based enzymes at acidic conditions for dehairing. This method leads to the solubilisation of collagen at the experimental conditions. Valeika et al (Journal of the Society of Leather Technologists and Chemists, 81, 65, 1997; 82, 95, 1998) have attempted to replace lime for dehairing using sodium hydroxide and sodium sulfide. They also found that the addition of salts such as sodium chloride, sodium sulfate, sodium formate or sodium hydrogen phosphate influence the extent of hair removal as well as opening up of the dermis structure. Commercial application of these methods is not popular in the global leather sector. Thanikaivelan et al (Journal of the Society of Leather Technologists and Chemists 84, 276, 2000) have developed a lime free enzymatic dehairing process along with reduced amount of sodium sulfide, which ensures complete dehairing within 18 hrs. However, enzyme-assisted lime-sulfide dehairing is being followed in some parts of the world. All the methods are applicable for only dehairing of skins/hides in leather processing. The dehaired pelts require fibre opening. Conventionally the fibre opening is obtained by treatment with lime through osmotic swelling.
Liming removes all the interfibrous materials especially proteoglycans and produces a system of fibres and fibrils of collagen which are clean as described by Campbell et al (Journal of American Leather Chemists Association, 68, 96, 1973). This is achieved by the alkali action as well as osmotic pressure built up in the skin matrix. Thanikaivelan et al (Environmental Science & Technology, 36, 4187, 2002) have successfully developed lime free fibre opening process employing x-amylase. However, no successful attempt has been made to eliminate lime and sodium sulfide completely in leather processing.
In our earlier application PCT/N03/00074, we have shown a novel process for an unhairing process using animal and/or herbal enzymes. The claimed process provides with a dime-sulphide free process for unhairing. The previously claimed method was restricted in the pH range of 4.0-10.0. In addition, the effect of use of silicate salt was not discussed and other parameters, which distinguishes our present work from the previous work. For example, identification and use of different enzymes and silicate salts for forming the paste and comparative study of the present invention with respect to the conventional lime-sulphide process. The present invention also makes an attempt to make a comparison between the quality of leather in the conventional method and present case.
Silicates have been widely used in various industrial applications for a long time. In leather manufacture, by contrast, the silica compounds have so far been of only minor importance. Wet-white tanning agent based on sodium aluminium silicate has been reported by Zauns and Kuhm (Journal of American Leather Chemists Association, 90, 177, 1995). Silicon dioxide based tanning system has been established by Fuchs and Kupfer (Journal of American Leather Chemists Association, 90, 164, 1995). Recently, Kanagaraj et al (Journal of American Leather Chemists Association, 95, 368, 2000) have developed a less salt preservation system based on silica gel and low amount of salt.